This invention relates to an electrophotographic apparatus employed in a printer, a copier and the like, and relates to a developing device that develops a latent image with a toner in the electrophotographic apparatus.
An electrophotographic apparatus employed in a printer, a copier and the like, has a developing device that develops a latent image formed on a photosensitive drum (i.e., a latent image bearing member) with a toner. There is a developing method called nonmagnetic single component development, in which a nonmagnetic toner is used and a carrier is not used. Developing devices applicable to this nonmagnetic single component development are disclosed, for example, in Japanese Laid-open Patent Publication Nos. HEI 7-44007, HEI 7-64394, HEI 9-80905, HEI 10-153910, HEI 12-98740 and HEI 13-56605.
FIG. 16 shows the developing device disclosed in Japanese Laid-open Patent Publication No. HEI 7-44007. The developing device includes a developing roller 100 that faces a photosensitive drum 110, and a supplying roller 111 that supplies the toner to the peripheral surface of the developing roller 100. A toner regulating roller 102 is disposed on the upper side of the developing roller 100 in FIG. 16. The toner regulating roller 102 regulates the thickness of the toner adhering to the peripheral surface of the photosensitive drum 110. A blade 103 contacts the peripheral surface of the toner regulating roller 102, for scraping the toner from the peripheral surface of the toner regulating roller 102. The toner, the thickness of which has been regulated by the toner regulating roller 102, is attracted by and adheres to a latent image formed on the photosensitive drum 110. After the toner on the developing roller 100 is transferred to the photosensitive drum 110, the residual toner (denoted by A) is removed by the recovering roller 104 disposed at the lower side of the developing roller 100 or the supplying roller 111. The developing devices disclosed in other publications have substantially the same basic constructions as that shown in FIG. 16.
In the conventional developing apparatus, the developing roller 100 carries the toner through a gap between the developing roller 100 and the toner regulating roller 102 to the photosensitive drum 110 side of the developing roller 100. Thus, in order to hold the toner layer on the peripheral surface of the developing roller 100, the friction coefficient of the peripheral surface of the developing roller 100 must be greater than that of the toner regulating roller 102. However, if the friction coefficient of the peripheral surface of the developing roller 100 is large, the releasability of the developing roller 100 against the toner (i.e., the easiness with which the toner is released from the developing roller 100) is low. Therefore, there is a case where the residual toner A on the peripheral surface of the developing roller 100 is not completely removed by the recovering roller 104 or the supplying roller 111. If the residual toner A remains on the peripheral surface of the developing roller 100, an additional toner adheres to the residual toner A when the developing roller 100 rotates once more, and the toner layer becomes partially thicker. The thicker part of the toner layer on the developing roller 100 may contact a part of the peripheral surface of the photosensitive drum 110 other than the latent image, so that an unintentional toner image is formed on the photosensitive drum 110. The unintentional toner image is transferred to a recording sheet, and forms a stain and the like (i.e., an afterimage) on the recording sheet, with the result that the quality of the image is degraded.
Further, the blade 103 is urged against the toner regulating roller 102 with a relatively large force, and may cause a so-called filming phenomenon where the toner is melted by the friction heat and adheres to the peripheral surface of the toner regulating roller 102 and the like. If such a filming phenomenon occurs, the thickness of the toner layer on the developing roller 100 becomes uneven, so that a stain or a defect in the image may be formed on the recording sheet, and therefore the quality of the image is degraded.
An object of the present invention is to provide a developing device and an electrophotographic apparatus capable of preventing degradation of the quality of an image.
According to the invention, there is provided a developing device including a developing member that develops a latent image formed on a latent image bearing member, a toner supplying member that supplies a toner to the developing member, and a toner layer forming member that has a peripheral surface contacting the developing member. The developing device further includes a toner layer regulating member that regulates the thickness of a toner layer that adheres to the peripheral surface of the toner layer forming member. The toner layer is transferred from the peripheral surface of the toner layer forming member to a peripheral surface of the developing member, after the thickness of the toner layer has been regulated by the toner layer regulating member.
With such an arrangement, the toner layer whose thickness has been regulated by the toner layer regulating member is transferred from the toner layer forming member to the developing member. Thus, even if the friction coefficient of the developing member is small, it is possible to uniformly form the toner layer having sufficient thickness on the peripheral surface of the developing member. As the peripheral surface of the developing member has a relatively small friction coefficient, it may increase the releasability of the developing member against the toner, and therefore it is ensured that the residual toner is thoroughly removed from the developing member. Thus, it is possible to prevent the occurrence of the afterimage caused by the residual toner on the developing member, and therefore it is possible to prevent the degradation of the quality of image.
Moreover, the toner layer regulating member is urged against the toner layer forming member with a relatively small force because it is unnecessary to completely remove the toner from the peripheral surface of the toner layer forming member. Thus, it is possible to prevent the occurrence of the filming phenomenon resulting from the melting of the toner due to the friction heat, and therefore the degradation of the quality of the image can be prevented. In addition, it is possible to vary the thickness of the toner layer formed on the peripheral surface of the developing member, by changing the friction coefficients, electric potentials and the like of the developing member and the toner layer forming member. As a result, the density of the printed image becomes variable.